1. Field of the Invention
The present invention relates to an aqueous intermediate coating composition and a method for forming a multilayer-coating film using the same.
2. Description of Related Art
Automotive bodies, especially exterior panel parts thereof, are usually coated with a multilayer coating film formed from an electrodeposition undercoat, an intermediate coat and a topcoat, to impart higher corrosion resistance and to improve appearance.
When a car is running, chipping occurs such that small stones, pebbles, antifreezing agents, small lumps of ice or like objects on the road are kicked up by the tires of the car and hit the exterior panel parts, thereby scratching the multilayer coating film, or even, in some extreme cases, locally damaging and chipping off the film. These phenomena cause problems such as harm to the external appearance, and exposure of the metal substrate which results in rusting and/or corrosion. In order to avoid these problems, an anti-chipping primer is often applied between the undercoat and the intermediate coat, which however is disadvantageous as it increases the number of coating steps. Accordingly, there is a demand for an intermediate coating composition that can form a multilayer coating film with higher chipping resistance without using an anti-chipping primer, thereby saving the number of coating steps.
Further, although organic solvent-based coating compositions are predominant among conventional intermediate coating compositions, an intermediate coating composition with an aqueous nature and a higher solids content is demanded to mitigate adverse effects on health and the environment, to reduce volatile organic compounds (VOC), and to improve coating efficiency.
JP 2004-43524 A discloses an aqueous intermediate coating composition comprising a graft polymer as a resin component, wherein the backbone polymer is a polyester resin and the branched polymer is an acrylic resin.
The viscosity of this intermediate coating composition is low even at a high solids content and thus is advantageous in that it can be applied in a high-solids-content state. However, this coating composition exhibits defects such that because its chipping resistance, a required property for an intermediate coating composition, is insufficient, when it is used without an anti-chipping primer, chipping resistance of the resulting coat is significantly lowered.
JP 2004-298836 A discloses a method for forming a coating film comprising the steps of applying, to a substrate whereon an electrodeposition coating layer has been formed, an aqueous intermediate coating composition comprising, together with a curing agent etc., an acryl resin emulsion having a glass transition temperature of −50 to 20° C., an acid value of 2 to 60 mgKOH/g and a hydroxy value of 10 to 120 mgKOH/g, obtained by emulsion polymerization of an alkyl(meth)acrylate, an acid group-containing ethylenic unsaturated monomer and a hydroxy-containing ethylenic unsaturated monomer; then an aqueous base coating composition; and then a clear coating composition, and simultaneously heat-curing the obtained three-layer coating film.
However, water resistance and like properties of a multilayer coating film obtained by this method are insufficient. Further, when, in this method, the intermediate coating layer is heat-cured before subsequent coating applications, the adhesion between the intermediate coating layer and the base coating layer is lowered, whereby chipping resistance of the multilayer coating film is also reduced.